As techniques for generating desired gaits of a mobile robot, such as a bipedal mobile robot, one disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-326173 (patent document 1) and one disclosed in PCT international publication WO/03/057427/A1(patent document 2) have been proposed by the present applicant. According to the techniques disclosed in these documents, an instantaneous desired gait composed of an instantaneous value of a desired motion (instantaneous desired motion) of a robot and an instantaneous value of a desired floor reaction force (instantaneous desired floor reaction force) is sequentially created using a first dynamic model (simplified model) representing a relationship between a motion of the robot (position and posture of each part) and a floor reaction force such that a dynamic balance condition (a condition, such as the one in which a translational force component of a floor reaction force reaches a desired value or a floor reaction force moment about a certain point reaches a desired value) on the first dynamic model is satisfied. Then, the instantaneous desired gait is input to a second dynamic model (full model) wherein a part of the instantaneous desired motion (desired body position/posture, a desired moment about a desired ZMP, or the like) is corrected so as to generate a final instantaneous desired gait in a time series manner. According to the technologies disclosed in documents 1 and 2, a desired gait is generated for each step, a gait for the period of one step of a bipedal mobile robot being the unit. In this case, to generate an instantaneous value of a desired gait for each step of the robot, the gait parameter (composed of the parameters defining the position/posture trajectory of each foot, the parameters defining a desired ZMP trajectory, etc.) that specifies a normal gait, which is a virtual cyclic gait that follows the desired gait, is first determined by using the gait parameter and the aforesaid first dynamic model such that a predetermined boundary condition (a condition in which the gait states at the start and the end, respectively, of one cycle of a normal gait coincide with each other) is satisfied. Further, the gait parameter defining a desired gait are determined such that a gait generated by using the gait parameter and the aforesaid first dynamic model satisfies a predetermined boundary condition (a condition in which the desired gait approaches the normal gait at the end side thereof). Then, the gait parameter of the desired gait determined as described above and the first dynamic model are used to generate the time series of the instantaneous values of the desired gait.
According to the technologies for generating desired gaits as described above, a model having high linearity is generally used as the first dynamic model (simplified model). Creating instantaneous desired gaits by using a dynamic model with high linearity make it possible to efficiently and promptly create a gait that connects to or gradually approximates a normal gait, which is a virtual cyclic gait (a gait that enables the robot to continue stable motions). As a result, instantaneous desired gaits of the robot can be sequentially generated in real time while performing actual motions of the actual robot. Moreover, when determining the gait parameter of a normal gait, it is possible to efficiently and promptly determine the gait parameter that allows the boundary condition of the normal gait to be satisfied.
However, a dynamic model with high linearity generally tends to have relatively low dynamic accuracy in a variety of operations of a robot. More specifically, the kinetics of a robot on its dynamic model is prone to errors relative to the actual kinetics of an actual robot. For this reason, if an instantaneous desired gait created using the first dynamic model is directly applied to the actual robot to make the actual robot operate, then a dynamic balance condition guaranteed on the first dynamic model will not be satisfied, frequently causing the operations of the actual robot to lack in stability.
Hence, in the technologies disclosed in the aforesaid patent documents 1 and 2, a part of the instantaneous desired gait created using the first dynamic model is further subjected to a correction by using a second dynamic model. In this case, a model whose dynamic accuracy is higher than that of the first dynamic model is used as the second dynamic model. This makes it possible to generate gaits of higher dynamic accuracy (closer to the dynamics of an actual robot) than that of gaits created using the first dynamic model.
However, in the technologies disclosed in the aforesaid patent documents 1 and 2, gaits created using the second dynamic model tend to diverge, so that it has been necessary to correct a desired ZMP trajectory or generate a floor reaction force moment about a desired ZMP so as to restrain a desired motion trajectory of a robot from deviating from a gait created using the first dynamic model. And, in this case, the correction amount of the desired ZMP trajectory or the floor reaction force moment about the desired ZMP occasionally becomes relatively large, and in such a case, it has become difficult to maintain an ample stability margin. Conversely, if the correction amount of the desired ZMP trajectory or the permissible range of the floor reaction force moment about the desired ZMP is set to be smaller in order to maintain an ample stability margin, then the likelihood of the divergence of a gait has inconveniently increased.
Further, when determining the gait parameter of a normal gait, if the dynamic accuracy of the first dynamic model is set to be high, then the linearity of the first dynamic model deteriorates, making it difficult to efficiently and promptly find a normal gait parameter that is capable of satisfying the boundary condition of the normal gait. Hence, it has been difficult to determine an appropriate normal gait parameter that is resistant to the divergence of a gait in the aforesaid second dynamic model. As a result, there has been an inconvenience in that maintaining an ample stability margin is difficult or gaits generated using the second dynamic model are apt to diverge, as described above.
The present invention has been made with a view of the above background, and it is an object thereof to provide a gait generating system for a mobile robot that is capable of efficiently generating desired gaits that allow a predetermined boundary condition to be satisfied with high dynamic accuracy, while preventing the desired gaits from diverging. Another object of the present invention is to provide a gait generating system for a mobile robot that is, when determining a gait parameter defining a normal gait, capable of efficiently determining a normal gait parameter that allows the normal gait to satisfy a predetermined boundary condition with high dynamic accuracy and also of preventing a desired gait from diverging.